Thousands of Indian farmers protesting against agricultural reforms on Tuesday overwhelmed the police and stormed into the historic Red Fort complex in New Delhi after tearing down barricades and driving tractors through roadblocks.

Farmers' leaders condemned the deadly violence, while stressing that they would not call off their protests.

Police fired tear gas in an unsuccessful bid to force the protesters back. One protester was killed, a witness said, and New Delhi police said 86 officers had been injured across the city.

Some of those who scaled the walls of the Red Fort carried ceremonial swords, scattering police who tried to prevent them from entering.

Angered by laws they say help large, private buyers at the expense of producers, farmers have camped outside the capital for almost two months, posing one of the biggest challenges to Prime Minister Narendra Modi since he came to power in 2014.

Last week the farmers rejected a government offer to put the laws on hold.

"Modi will hear us now, he will have to hear us now," said Sukhdev Singh, 55, a farmer from the northern state of Punjab.

The body of one protester draped in an Indian tricolour lay in the street after the tractor he rode overturned in a clash, said a witness, Vishu Arora.

"He died right there," Arora said.

The government ordered internet services in some parts of the capital to be blocked, according to mobile carrier Vodafone Idea, in an attempt to prevent further unrest.

The violence marred Republic Day Celebrations, a national holiday marking the anniversary of India officially adopting its constitution on January 26, 1950.

Protest organizer Samyukt Kisan Morcha said the groups deviating from set routes did not represent the majority of farmers.

"We also condemn and regret the undesirable and unacceptable events that have taken place today and dissociate ourselves from those indulging in such acts," the group of farm unions said in a statement.

Amarinder Singh, chief minister of Punjab state, where many of the protesters came from, called the clashes "shocking."

"The violence by some elements is unacceptable," he said in a tweet. "It'll negate goodwill generated by peacefully protesting farmers."

(With input from Reuters)